[Bldg-sim] TRANSFER FUNCTION FOR HEAVY AND THICK WALLS

[Joe Huang]

Luis,

I'm not sure if I have an answer for your problem, but I want to point out some technical 
details that may be being overlooked.

1. DOE-2 does not use CTFs, it uses Response Factors, which shares the same approach as 
CTFs but uses a different formulation.

2. When DOE-2 says it cannot simulate a layer because it's too thick, that's not because 
the RF methodology fails but because DOE-2 has a maximum of 100 time-steps, i.e., 100 
hours, for the three response factors (X,Y,Z) to converge to constant COMMON RATIOs that 
are used to  compute the residual responses from that point on.

3. There are two situations that could cause such convergence problems - (a) the layer is 
simply too massive. I know from trial and error that the thickest single-layer of dirt 
allowed in DOE-2 is 5.25 ft (1.6 meters), (b) this thickness will be greatly reduced if 
there are multiple layers with contrasting thermal properties; again, from 
trial-and-error, if there is a layer of insulation below the dirt, the maximum allowable 
thickness of the dirt layer will be reduced by half or more.  You've mentioned that 
someone has mentioned creating multiple thin layers. This will do help with (a), but could 
help with (b), provided that the thin layers are graduated in their thermal properties; 
I've never tried this so I can't tell how much benefit would there be, but my guess is not 
very much.

3. I don't know EnergyPlus well enough to say whether it has a similar hard limit on 
convergence and if so, what is it.  If not, you might be able to model the same layer in 
EnergyPlus and not encounter any difficulties.

4. If all else fails, as others have already mentioned, you can do an explicit solution 
using the Finite Difference Method in EnergyPlus, but expect that to slow down your 
simulations considerably.

Joe

Joe Huang
White Box Technologies, Inc.
346 Rheem Blvd., Suite 108D
Moraga CA 94556
yjhuang at whiteboxtechnologies.com
http://weather.whiteboxtechnologies.com for simulation-ready weather data
(o) (925)388-0265
(c) (510)928-2683
"building energy simulations at your fingertips"


On 5/16/2014 6:37 AM, Luis Miguel Blanes Restoy wrote:
> Luis
>
> You may explore E+ different ways of dealing with CTF (state space method mainly). CTF 
> calculations become unstable for short time steps and massive constructions (it is at 
> the end a computational problem). E-Quest may decide on your arrangement to be unstable 
> before running the simulation (not sure of this). There are solutions to solve this 
> problem (Seem 1987), and I guess these have been incorporated into CTF calculation 
> within E+. It is a matter of giving E+ a go and try out if your example may work there.
>
> E+ also handles Finite Differences Methods for CTF calculations, but I am not versed in 
> this field.
>
> You can find more information in the E+ Engineering Reference Manual.
>
> Regards
> Abrazos desde Hibernia
> Luis
>
>
>
>
> 2014-05-16 14:06 GMT+01:00 Luis Peréz-Lombard <lpl at us.es <mailto:lpl at us.es>>:
>
>     Dear colleagues,
>
>     As you probably know, Conduction Transfer Function (CTF) method does not handle
>     thick and massive walls, because the accuracy of the CTF coefficients is degraded.
>     In particular, in DOE-2, you get the following message:
>
>     /“Cannot simulate LAYERS …….. as given. The wall is too thick or too dense”/
>
>     I have heard of a possible solution to solve this problem by dividing the wall into
>     thin layers. I have made an attempt in e-quest, but the problem remains, and I do
>     not know how to proceed.
>
>     Any insight on this issue would be appreciated.
>
>     Sincerely,
>
>     LPL
>
>     -------------------------------------------------------------
>     Luis Pérez-Lombard
>     Dr. Ingeniero Industrial
>     Profesor Contratado Doctor
>     Departamento de Ingeniería Energética
>     Escuela Superior de Ingenieros
>     Universidad de Sevilla
>     Camino de los Descubrimientos s/n
>     41092 SEVILLA
>     Tfno.:      95.448.72.56
>     Fax:         95.446.31.53
>     e-mail: lpl at us.es <mailto:lpl at us.es>
>     -------------------------------------------------------------
>
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>
>
> -- 
> LUIS M BLANES RESTOY
> *IRUSE
> *INFORMATICS RESEARCH UNIT FOR SUSTAINABLE ENGINEERING
> www.iruse.ie <http://www.iruse.ie>
> mobile Ireland: +353 83 1346875
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>
>
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